Weigh-in-Motion, briefly WIM, systems are used in the field of transport for determining the total load or the axle load of a vehicle or for determining the number of crossings of axles or vehicles over a roadway segment. For this purpose, a WIM system is not limited to consisting of at least a WIM sensor and at least an evaluation element. A roadway segment refers to a roadway comprising a road surface, a road sublayer as well as an area alongside the roadway. On the basis of the detected numbers and/or axle loads of vehicles measures may be initiated which are inter alia appropriate for preventing a damage to the roadway due to overload, determining use-dependent maintenance intervals for the roadway, determining payments depending on the total load or the axle load or the number of axles of a vehicle, and increasing traffic safety.
WIM sensors are embedded in the road surface of a roadway segment and in a longitudinal direction with respect to the direction of travel a track of a vehicle is crossed by several WIM sensors that are inserted in the road surface next to each other in the direction of travel so that one WIM sensor each crosses a wheel track of a vehicle wherein the WIM sensor is arranged in the surface of the roadway segment flush with the road surface of that roadway segment. Generally, the WIM sensor is formed as an elongated profile. The wheel track refers to the trajectory of a wheel of the vehicle on the road surface when the vehicle traverses. By track is meant the totality of all wheel tracks of the vehicle. Also, an embodiment is known in which the complete track of the vehicle is crossed by a single elongated WIM sensor.
Usually, at least one force sensor is arranged in the WIM sensor for determining the load of a wheel or axle. Upon application of a force, for example due to a wheel load of a crossing wheel, the force sensor outputs a force sensor signal.
The determination of the load of a wheel is performed by measuring dynamic ground reaction forces when a wheel crosses by means of at least one force sensor and by measuring the speed of a wheel. The ground reaction force means the force that counteracts a force exerted by a wheel on the ground. The speed is usually calculated on the basis of the transit time between two WIM sensors spaced apart in the direction of travel of the wheel.
The determination of the load of the wheel is falsified due to a resonance of the road surface and/or road sublayer in which the WIM sensor is inserted. Thus, a soft road surface or road sublayer will result in a strong resonance of the road surface or road sublayer so that a lower force will be detected compared to a firmer, rigid underground or road surface. Hereinafter, the resonance of the underground will be referred to as deflection wherein the deflection indicates how strongly the road surface or road sublayer is deflected from a resting position, said resting position being the position of the road sublayer or road surface without any load acting thereon.
Furthermore, also the costs of a WIM system play an increasingly important role. It is demanded that the maintenance intervals will be as long as possible and, as a result of the trend of interconnecting a plurality of WIM systems, that the WIM sensors will function as autonomously as possible.
A WIM sensor is known from U.S. Pat. No. 5,265,481A, which is hereby incorporated herein by this reference for all purposes, and includes a plurality of piezoelectric measuring elements, briefly referred to as piezoelements, arranged in a hollow profile in a longitudinal direction; wherein the piezoelements are connected to a signal processing unit so that it is possible to electrically contact an individual piezoelement or a group of piezoelements in parallel; wherein a piezoelement consists of piezo discs arranged between force introduction segments. In the hollow profile laterally next to a piezoelement may be arranged electronic components, such as a preamplifier, which contact the piezoelement or the group of piezoelements. This allows adapting of the local measurement sensitivity of the associated piezoelement or -elements by means of the preamplifier to the environment in which the WIM sensor is arranged. Due to this adaptation, also referred to as calibration, the measurement accuracy of a WIM sensor is increased as compared to a calibration after production without considering the environment in which the WIM sensor will be arranged.
A disadvantage is, however, that the calibration is performed by a calibration vehicle and must be repeated in regular intervals in order to restore the original measurement accuracy.
In real operation, the measurement accuracy of the WIM sensor decreases continuously due to a change in the deflection of the road surface and the road sublayer that occurs in normal operation, for example due to aging, wear or changes in the temperature of the road surface or road sublayer.